Air pollution control system, air pollution control method, spray drying device of dewatering filtration fluid from desulfurization discharged water, and method thereof

ABSTRACT

To include a boiler  11  that burns fuel F, an air heater  13  that recovers heat of flue gas  18  from the boiler  11,  a first precipitator  14  that reduces dust in the flue gas  18  after heat recovery, a desulfurizer  15  that reduces sulfur oxides in the flue gas  18  after dust reduction by an absorbent, a dewaterer  32  that reduces gypsum  31  from desulfurization discharged water  30  discharged from the desulfurizer  15,  a spray drying device  34  including an atomizer that atomizes a dewatering filtration fluid  33  discharged from the dewaterer  32,  and a flue-gas introducing line L 11  that introduces a part of the flue gas  18  into the spray drying device  34.

CROSS-REFERENCE TO RELATED APPLICATION

This Application is a Divisional of copending U.S. application Ser. No.13/149,328 filed May 31, 2011, which is based upon and claims priorityunder 35 U.S.C. §119 (a) to Japan Patent Application No. 2011-063363filed Mar. 22, 2011 and Japan Patent Application No. 2011-071005 filedMar. 28, 2011.

TECHNICAL FIELD

The present invention relates to an air pollution control system thattreats flue gas discharged from a boiler, an air pollution controlmethod, a spray drying device of a dewatering filtration fluid fromdesulfurization discharged water, and a method thereof.

BACKGROUND ART

Conventionally, there has been known an air pollution control system fortreating flue gas discharged from a boiler installed in a thermal powergeneration plant or the like. The air pollution control system includesNO_(x) removal equipment that removes nitrogen oxides from flue gasdischarged from a boiler, an air heater that recovers heat of flue gashaving passed through the NO_(x) removal equipment, a precipitator thatreduces dust in the flue gas after heat recovery, and a desulfurizerthat reduces sulfur oxides in the flue gas after dust reduction. As thedesulfurizer, a wet desulfurizer that reduces sulfur oxides in flue gasby bringing a lime absorbent into gas-liquid contact with flue gas hasbeen generally used.

Waste water discharged from a wet desulfurizer (hereinafter,“desulfurization discharged water”) contains various types of harmfulsubstances, for example, ions such as chlorine ion and ammonium ion andmercury in large amount. Therefore, these harmful substances need to beremoved from the desulfurization discharged water before thedesulfurization discharged water is discharged to outside of the system.However, a removing process of these various types of harmful substancescontained in the desulfurization discharged water is complicated, andtreatment cost is high. Therefore, to reduce the treatment cost of thedesulfurization discharged water, there has been proposed a method ofreusing the desulfurization discharged water in the system withoutdischarging it to the outside of the system. For example, PatentLiterature 1 discloses an air pollution control system in which a devicethat atomizes and gasifies desulfurization discharged water isseparately installed, branched from a flue gas duct of a main line thatconnects NO_(x) removal equipment, an air heater, a precipitator, and adesulfurizer, and after a part of flue gas is introduced from the fluegas duct of the mainline into the device, and desulfurization dischargedwater is atomized into flue gas in the device and evaporated toprecipitate harmful substances, the flue gas is returned to the flue gasduct of the main line (Patent Literatures 1 and 2).

CITATION LIST Patent Literature

[PTL 1] JP S63-200818A

[PTL 2] JP H9-313881A

SUMMARY OF INVENTION Technical Problem

In the air pollution control systems described in Patent Literatures 1and 2, a device that partially branches flue gas from a flue gas duct,and atomizes desulfurization discharged water (or effluent) from adesulfurizer to effect gasification is provided to evaporatedesulfurization discharged water. However, because desulfurizationdischarged water from the desulfurizer contains solid contents, spraydrying cannot be performed satisfactorily.

Furthermore, in recent years, zero liquid discharge in air pollutioncontrol systems has been desired due to the environmental concerns withrespect to water resources in inland areas and the like, and there hasalso been desired an air pollution control system that can promote zeroliquid discharge to ensure stable operations.

As a device for implementing zero liquid discharge, a spray dryingdevice that dries desulfurization discharged water can be used. However,when desulfurization discharged water is spray-dried, there are thefollowing problems.

-   1) Problem Caused by Disturbance in Balance of Heat Quantity

To evaporate spray liquid, drying is promoted by heat transfer betweenthe spray liquid and hot air; however, when spray liquid is excessivewith respect to hot air, insufficient evaporation occurs.

-   2) Problem Caused by Coarsening of Droplet Size of Spray Liquid Due    to Ash Deposition

When ash is deposited at an end of a spray nozzle, the droplet size ofspray liquid emitted from the nozzle changes, and generally coarseningof the droplet size occurs. The coarsened droplet has a small specificsurface area for heat exchange with hot air, and heat exchange becomesslow, thereby causing evaporation delay.

The present invention has been achieved in view of the above problems,and an object of the present invention is to provide an air pollutioncontrol system that promotes zero liquid discharge to ensure stableoperations, an air pollution control method, a spray drying device of adewatering filtration fluid from desulfurization discharged water, and amethod thereof.

Solution to Problem

According to an aspect of the present invention, an air pollutioncontrol system includes: a boiler that burns fuel; an air heater thatrecovers heat of flue gas from the boiler; a first precipitator thatreduces dust in the flue gas after heat recovery; a desulfurizer thatreduces sulfur oxides in the flue gas after dust reduction by anabsorbent; a dewaterer that reduces gypsum from desulfurizationdischarged water discharged from the desulfurizer; a spray drying deviceincluding an atomizer that atomizes a dewatering filtration fluiddischarged from the dewaterer; and a flue-gas introducing line thatintroduces a part of the flue gas into the spray drying device.

Advantageously, the air pollution control system further includes asolid-liquid separating unit that reduces suspended solids in adewatering filtration fluid from the dewaterer.

Advantageously, the air pollution control system further includes adewatering branch line that supplies a dewatering filtration fluid fromthe dewaterer to collected dust.

Advantageously, in the air pollution control system, the spray dryingdevice is a solid-gas separating spray drying device.

Advantageously, the air pollution control system further includes awaste-water treatment device that reduces harmful substances in adewatering filtration fluid discharged from the dewaterer.

Advantageously, in the air pollution control system, a secondprecipitator is provided either on an upstream side or a downstream sideof the spray drying device provided in the flue-gas introducing line oron both sides.

Advantageously, in the air pollution control system, a branchingposition of the flue gas is on an upstream side of the air heater, andthe flue gas from the spray drying device is returned to between the airheater and the first precipitator.

Advantageously, in the air pollution control system, a branchingposition of the flue gas is on an upstream side of the air heater, andthe flue gas from the spray drying device is returned to between the airheater and the first precipitator or to a downstream side of the firstprecipitator.

Advantageously, in the air pollution control system, a branchingposition of the flue gas is between the air heater and the firstprecipitator, and flue gas from the spray drying device is returned tobetween the air heater and the first precipitator.

Advantageously, in the air pollution control system, a branchingposition of the flue gas is between the air heater and the firstprecipitator, and the flue gas from the spray drying device is returnedto between the air heater and the first precipitator or to a downstreamside of the first precipitator.

Advantageously, in the air pollution control system, a branchingposition of the flue gas is between the first precipitator and thedesulfurizer, and the flue gas from the spray drying device is returnedto between the air heater and the first precipitator or to a downstreamside of the first precipitator.

Advantageously, in the air pollution control system, a branchingposition of the flue gas is between the first precipitator and thedesulfurizer, and the flue gas from the spray drying device is returnedto the first precipitator and the desulfurizer.

According to another aspect of the present invention, in an airpollution control method, after heat of flue gas from a boiler thatburns fuel is recovered by an air heater, sulfur oxides contained in theflue gas after heat recovery are reduced by an absorbent in adesulfurizer, and a dewatering filtration fluid acquired by reducinggypsum from desulfurization discharged water discharged from thedesulfurizer is spray-dried by a part of the flue gas.

Advantageously, in the air pollution control method, a separate liquidin which suspended solids in the dewatering filtration fluid are reducedis spray-dried.

Advantageously, in the air pollution control method, solids are reducedfrom the flue gas used for spray drying.

According to another aspect of the present invention, a spray dryingdevice of a dewatering filtration fluid from desulfurization dischargedwater, includes: a spray nozzle that atomizes a dewatering filtrationfluid from desulfurization discharged water into a spray drying devicebody; an introduction port provided on the spray drying device body tointroduce flue gas for drying a spray liquid; a dry area provided in thespray drying device body to dry a dewatering filtration fluid by fluegas; a discharge port that discharges the flue gas having contributed todrying; and a deposit monitor that monitors an attached state of adeposit to the spray nozzle.

Advantageously, in the spray drying device of a dewatering filtrationfluid from desulfurization discharged water, the deposit monitormonitors a growth state of ash deposit by using ultrasonic waves or alaser.

Advantageously, the spray drying device of a dewatering filtration fluidfrom desulfurization discharged water further includes a depositremoving unit that removes the deposit.

Advantageously, in the spray drying device of a dewatering filtrationfluid from desulfurization discharged water, the deposit removing unitis a scraper movably provided on an outer circumference of the spraynozzle.

Advantageously, in the spray drying device of a dewatering filtrationfluid from desulfurization discharged water, the deposit removing unitis a spray-nozzle cleaning unit.

Advantageously, the spray drying device of a dewatering filtration fluidfrom desulfurization discharged water further includes: thermometersprovided in plural in a dry area to measure an internal temperature; adetermining unit that determines whether a spray-dried state of adewatering filtration fluid is favorable based on measurement results ofthe thermometers; and a control unit that adjusts the flue gas or thedewatering filtration fluid when it is determined that spray drying isnot sufficient based on a determination made by the determining unit.

According to still another aspect of the present invention, an airpollution control system includes: a boiler that burns fuel; an airheater that recovers heat of flue gas from the boiler; a precipitatorthat reduces dust in the flue gas after heat recovery; a desulfurizerthat reduces sulfur oxides in the flue gas after dust reduction by anabsorbent; a dewaterer that reduces gypsum from desulfurizationdischarged water discharged from the desulfurizer; the spray dryingdevice of claim 16 including an atomizer that atomizes a dewateringfiltration fluid discharged from the dewaterer; and a flue-gasintroducing line that introduces a part of the flue gas into the spraydrying device.

Advantageously, the air pollution control system further includes asolid-liquid separating unit that reduces suspended solids in thedewatering filtration fluid discharged from the dewaterer.

According to still another aspect of the present invention, in a spraydrying method of a dewatering filtration fluid from desulfurizationdischarged water, a dewatering filtration fluid from desulfurizationdischarged water is atomized into a spray drying device body and a sprayliquid is dried by introduced flue gas, and an atomization state of aspray nozzle is confirmed to determine whether atomization of thedewatering filtration fluid is appropriate, and when the atomization isinappropriate, the spray nozzle is cleaned and ash deposit attachedaround the spray nozzle is removed. That is, the method may includeatomizing a dewatering filtration fluid with a spray nozzle; introducingflue gas so as to dry the dewatering filtration fluid; confirming anatomization state of a spray nozzle and determining whether atomizationof the dewatering filtration fluid is appropriate; and cleaning thespray nozzle and removing ash deposit attached around the spray nozzlewhen the atomization is inappropriate.

Advantageously, in the spray drying method of a dewatering filtrationfluid from desulfurization discharged water, a temperature distributioninside the spray drying device body is measured, a dried state ismonitored by a temperature distribution in a direction of a gas flow,and when drying of the dewatering filtration fluid is not sufficient,supply amounts of flue gas and of the dewatering filtration fluid areadjusted. That is, the method may include measuring a temperaturedistribution inside the spray drying device body in a direction of a gasflow; monitoring a dried state with the temperature distribution; andadjusting supply amounts of the flue gas and of the dewateringfiltration fluid, when drying of the dewatering filtration fluid is notsufficient.

Advantageous Effects of Invention

According to the air pollution control system and the air pollutioncontrol method of the present invention, the dewatering filtration fluidacquired by removing gypsum from the desulfurization discharged waterseparated from the desulfurizer is atomized by the spray drying device,by using flue gas from the boiler. Therefore, spray drying can be stablyperformed, thereby enabling to realize zero liquid discharge of thedesulfurization discharged water from the desulfurizer.

According to the present invention, further, at the time of atomizationby the spray drying device by using the desulfurization discharged wateracquired by removing gypsum from the desulfurization discharged waterseparated from the desulfurizer, stable atomization can be performedwhile ascertaining a spray-dried state, and reducing the deposit whenthere is insufficient atomization. Accordingly, zero liquid discharge ofthe desulfurization discharged water from the desulfurizer can berealized.

BRIEF DESCRIPTION OF DRAWINGS

FIG. 1 is a schematic configuration diagram of an air pollution controlsystem according to a first embodiment of the present invention.

FIG. 2 is a schematic configuration diagram of another air pollutioncontrol system according to the first embodiment.

FIG. 3 is a schematic configuration diagram of an air pollution controlsystem according to a second embodiment of the present invention.

FIG. 4A is a schematic configuration diagram of an air pollution controlsystem according to a third embodiment of the present invention.

FIG. 4B is a schematic configuration diagram of another air pollutioncontrol system according to the third embodiment.

FIG. 4C is a schematic configuration diagram of still another airpollution control system according to the third embodiment.

FIG. 5A is a schematic configuration diagram of an air pollution controlsystem according to a fourth embodiment of the present invention.

FIG. 5B is an example of a solid-gas separating spray drying device.

FIG. 5C is another example of a solid-gas separating spray dryingdevice.

FIG. 6 is a schematic configuration diagram of an air pollution controlsystem according to a fifth embodiment of the present invention.

FIG. 7 is a schematic configuration diagram of an air pollution controlsystem according to a sixth embodiment of the present invention.

FIG. 8A is a schematic configuration diagram of another air pollutioncontrol system according to the sixth embodiment.

FIG. 8B is a schematic configuration diagram of still another airpollution control system according to the sixth embodiment.

FIG. 8C is a schematic configuration diagram of still another airpollution control system according to the sixth embodiment.

FIG. 9 is a schematic configuration diagram of an air pollution controlsystem according to a seventh embodiment of the present invention.

FIG. 10 is a schematic configuration diagram of another air pollutioncontrol system according to the seventh embodiment.

FIG. 11 is a schematic diagram of a spray drying device of a dewateringfiltration fluid from desulfurization discharged water according to aneighth embodiment of the present invention.

FIG. 12A is a schematic diagram of a monitoring state of a deposit by adeposit monitor.

FIG. 12B is a schematic diagram of a monitoring state of a deposit bythe deposit monitor.

FIG. 12C is a schematic diagram of a monitoring state of a deposit bythe deposit monitor.

FIG. 13 is a schematic diagram of another spray drying device of adewatering filtration fluid from desulfurization discharged wateraccording to the eighth embodiment.

FIG. 14A depicts a state of removing a deposit by a scraper providedaround a spray nozzle.

FIG. 14B depicts a state of removing a deposit by the scraper providedaround the spray nozzle.

FIG. 14C depicts a state of removing a deposit by the scraper providedaround the spray nozzle.

FIG. 15 is a schematic diagram of a spray nozzle according to a ninthembodiment of the present invention.

FIG. 16 is a schematic diagram of a spray drying device according to atenth embodiment of the present invention.

FIG. 17A is a relationship diagram between a distance from a nozzle toseven thermometers (T₁ to T₇) in a dryer body and measured temperatures.

FIG. 17B is a relationship diagram between a distance from a nozzle tothe seven thermometers (T₁ to T₇) in the dryer body and measuredtemperatures.

DESCRIPTION OF EMBODIMENTS

Exemplary embodiments of the present invention will be explained belowin detail with reference to the accompanying drawings. The presentinvention is not limited to the embodiments, and when there are aplurality of embodiments, embodiments that are constituted by combiningthese embodiments are also included in the scope of the invention. Inaddition, constituent elements in the following embodiments includethose that can be easily assumed by persons skilled in the art or thatare substantially equivalent.

First Embodiment

FIG. 1 is a schematic configuration diagram of an air pollution controlsystem according to a first embodiment of the present invention. An airpollution control system 10A exemplified in FIG. 1 removes harmfulsubstances such as nitrogen oxides (NO_(x)), sulfur oxides (SO_(x)), andmercury (Hg) from flue gas 18 discharged from a boiler 11 such as a coalcombustion boiler that uses coals as a fuel or a heavy-oil combustionboiler that uses heavy oil as a fuel.

The air pollution control system 10A includes the boiler 11 thatcombusts fuel F, an air heater 13 that recovers heat of the flue gas 18from the boiler 11, a first precipitator 14 that reduces dust in theflue gas 18 after heat recovery, a desulfurizer 15 that reduces sulfuroxides in the flue gas 18 after dust reduction by a lime slurry 20,which is an absorbent, a dewaterer 32 that reduces gypsum 31 fromdesulfurization discharged water 30 discharged from the desulfurizer 15,a spray drying device 34 including an atomizer that atomizes adewatering filtration fluid 33 from the dewaterer 32, and a flue-gasintroducing line L₁₁ that introduces a part of the flue gas 18 into thespray drying device 34. Accordingly, because spray drying is performedby the spray drying device 34 by using the dewatering filtration fluid33 in which the gypsum 31 is reduced, stable atomization can beperformed.

Accordingly, clogging in the spray drying device 34 does not occur, andzero liquid discharge of a moisture content of the desulfurizationdischarged water can be stably performed.

NO_(x) removal equipment 12 removes nitrogen oxides in the flue gas 18supplied from the boiler 11 via a gas supply line L₁, and includes anNO_(x) removal catalyst layer (not shown) therein. A reducing agentinjector (not shown) is arranged on the upstream of the NO_(x) removalcatalyst layer, and a reducing agent is injected into the flue gas 18from the reducing agent injector. As the reducing agent, for example,ammonia, urea, ammonium chloride are used. The flue gas 18 introducedinto the NO_(x) removal equipment 12 comes in contact with the NO_(x)removal catalyst layer, and nitrogen oxides in the flue gas 18 aredecomposed into nitrogen gas (N₂) and water (H₂O) and removed. When achlorine (Cl) content in the flue gas 18 increases, the proportion of abivalent mercury chloride soluble in water increases, and mercury can beeasily collected by the desulfurizer 15 described later.

The NO_(x) removal equipment 12 is not essential, and when aconcentration of nitrogen oxides or mercury in the flue gas 18 from theboiler 11 is very small or these substances are not contained in theflue gas 18, the NO_(x) removal equipment 12 can be omitted.

The air heater 13 is a heat exchanger that recovers heat in the flue gas18, which is supplied via a gas supply line L₂, after nitrogen oxideshave been removed by the NO_(x) removal equipment 12. Because thetemperature of the flue gas 18 having passed through the NO_(x) removalequipment 12 is as high as about 350° C. to 400° C., the air heater 13performs heat exchange between the high-temperature flue gas 18 andcombustion air at a normal temperature. Combustion air, which becomes ahigh temperature by heat exchange, is supplied to the boiler 11. On theother hand, the flue gas 18 having been heat-exchanged with combustionair at a normal temperature is cooled to about 150° C.

The first precipitator 14 reduces dust in the flue gas 18, which issupplied via a gas supply line L₃, after heat recovery. As the firstprecipitator 14, an inertial precipitator, a centrifugal precipitator, afiltering precipitator, an electric precipitator, and a cleaningprecipitator can be mentioned; however, it is not particularly limitedthereto.

The desulfurizer 15 reduces sulfur oxides in the flue gas 18, which issupplied via a gas supply line L₄, after dust reduction according to awet method. In the desulfurizer 15, the lime slurry 20 (a solution inwhich limestone powder is dissolved in water) is used as an alkalineabsorbent, and the temperature inside the desulfurizer is adjusted toabout 30° C. to 80° C. The lime slurry 20 is supplied from a lime-slurrysupply system 21 to a column bottom part 22 of the desulfurizer 15. Thelime slurry 20 supplied to the column bottom part 22 of the desulfurizer15 is supplied to a plurality of nozzles 23 in the desulfurizer 15 viaan absorbent supply line (not shown), and is ejected from the nozzles 23toward a column top part 24 of the desulfurizer 15. Because the flue gas18 rising from the column bottom part 22 of the desulfurizer 15 comes ingas-liquid contact with the lime slurry 20 ejected from the nozzles 23,sulfur oxides and mercury chloride in the flue gas 18 are absorbed bythe lime slurry 20, and separated and removed from the flue gas 18. Theflue gas 18 purified by the lime slurry 20 is discharged from the columntop part 24 of the desulfurizer 15 as purged gas 26, and discharged tooutside of the system from a stack 27.

At the inside of the desulfurizer 15, sulfur oxides SO_(X) in the fluegas 18 causes a reaction with the lime slurry 20 represented by thefollowing expression (1).

CaCO₃+SO₂+0.5H₂O→CaSO₃.0.5H₂O+CO₂  (1)

The lime slurry 20 that has absorbed SO_(X) in the flue gas 18 is thenoxidized by air (not shown) supplied to the column bottom part 22 of thedesulfurizer 15, to cause a reaction with air represented by thefollowing expression (2).

CaSO₃.0.5H₂O+0.5O₂+1.5H₂O→CaSO₄.2H₂O  (2)

In this manner, SO_(x) in the flue gas 18 is captured in a state ofgypsum CaSO₄.2H₂O in the desulfurizer 15.

As described above, while a solution accumulated in the column bottompart 22 of the desulfurizer 15 of a wet type and pumped is used as thelime slurry 20, gypsum CaSO₄.2H₂O is mixed in the lime slurry 20 to bepumped by an operation of the desulfurizer 15, according to the abovereaction expressions (1) and (2). The lime gypsum slurry (a lime slurrymixed with gypsum) to be pumped is hereinafter referred to as“absorbent”.

The absorbent (the lime gypsum slurry) used for desulfurization isdischarged to outside from the column bottom part 22 of the desulfurizer15 as the desulfurization discharged water 30, and supplied to thedewaterer 32 via a waste water line L₂₀ described later, wheredewatering is performed. As well as gypsum 31, heavy metal such asmercury and halogen ions such as Cl⁻, Br⁻, I⁻, and F⁻ are included inthe desulfurization discharged water 30.

The dewaterer 32 separates a solid content including the gypsum 31 inthe desulfurization discharged water 30 and a liquid content of thedewatering filtration fluid 33. As the dewaterer 32, for example, a beltfilter, a centrifugal separator, or a decanter-type centrifugal settleris used. The gypsum 31 in the desulfurization discharged water 30discharged from the desulfurizer 15 is separated by the dewaterer 32. Atthis time, mercury chloride in the desulfurization discharged water 30is separated from the liquid together with the gypsum 31 in a state ofbeing adsorbed on the gypsum 31 . The separated gypsum 31 is dischargedto outside of the air pollution control system (hereinafter, “outside ofthe system”).

On the other hand, the dewatering filtration fluid 33, which is separateliquid, is supplied to the spray drying device 34 via a dewatering lineL₂₁. Alternatively, the dewatering filtration fluid 33 can betemporarily stored in a discharged water tank (not shown).

The spray drying device 34 includes a gas introducing unit into which apart of the flue gas 18 is introduced via the flue-gas introducing lineL₁₁ branched from the gas supply line L₂, and an atomizer that sprays oratomizes the dewatering filtration fluid 33. The spray drying device 34evaporates and dries the sprayed dewatering filtration fluid 33 by theheat of the introduced flue gas 18.

In the present invention, because the dewatering filtration fluid 33acquired by removing the gypsum 31 from the desulfurization dischargedwater 30 is spray-dried, clogging in the atomizer can be prevented.

That is, because the desulfurization discharged water 30 is not directlyatomized, an amount of dried particles generated due to evaporation ofthe desulfurization discharged water 30 can be considerably reduced. Asa result, clogging caused by attachment of dried particles can bedecreased. Further, because mercury chloride can be also separated andremoved together with the gypsum 31 by dewatering the desulfurizationdischarged water 30, it can be prevented that a mercury concentration inthe flue gas 18 increases at the time of atomizing discharged water.

In the first embodiment, because apart of flue gas flowing into the airheater 13 is branched from the gas supply line L₂ via the flue-gasintroducing line L₁₁, the temperature of flue gas is high (350° C. to400° C.), and spray drying of the dewatering filtration fluid 33 can beefficiently performed.

FIG. 2 is a schematic configuration diagram of another air pollutioncontrol system according to the first embodiment.

In an air pollution control system 10B shown in FIG. 2, a part of thedewatering filtration fluid 33 is atomized to collected dust 16discharged from the first precipitator 14 via a dewatering branch lineL₂₂ branched from the dewatering line L₂₁. A moisture content of thecollected dust 16 after being atomized and mixed is preferably 15% at amaximum.

Accordingly, a part of the dewatering filtration fluid 33 can be reducedwithout performing spray drying.

In the collected dust 16 containing moisture, scattering of ash isprevented, to improve handling in ash disposal. Conventionally,industrial water in a facility has been atomized. Therefore, the costfor industrial water is not required, thereby enabling economicaltreatment.

Second Embodiment

An air pollution control system according to a second embodiment of thepresent invention is explained next. Constituent elements identical tothose in the first embodiment described above are denoted by likereference signs, and explanations thereof will be omitted. FIG. 3 is aschematic configuration diagram of the air pollution control systemaccording to the second embodiment. In an air pollution control system10C according to the second embodiment, as shown in FIG. 3, asolid-liquid separating unit 41 that reduces suspended solids (SS) orsuspended substance in the dewatering filtration fluid 33 is installedin the dewatering line L₂₁.

As the solid-liquid separating unit 41, for example, a hydrauliccyclone, a belt filter, a classifier, or a membrane separator can bementioned.

The solid-liquid separating unit 41 reduces suspended solids (SS) in thedewatering filtration fluid 33 so that an SS concentration in a separateliquid 42 becomes equal to or less than 1% by weight, more preferably,from 0.1% to 0.5% by weight.

Accordingly, the SS concentration decreases, thereby enabling to furthersuppress clogging of nozzles and pipes in the spray drying device 34.

That is, by decreasing the SS concentration to a level equal to or lessthan 1% by weight, and more preferably, of from 0.1% to 0.5% by weight,insufficient atomization caused by attachment of atomized dry substancesor attachment and growth of dust at the ends of the spray nozzles at thetime of spray drying can be suppressed. As a result, such problems asshutdown due to blockage, and insufficient drying resulting from longdrying time to be required due to coarsening of the droplet size ofspray liquid can be resolved. Further, nonuniform drying andinsufficient drying caused by a bias of an atomization range areresolved.

A separate residue 43 separated by the solid-liquid separating unit canbe joined to the collected dust 16, so that moisture is containedtherein by the dewatering filtration fluid 33.

When the collected dust 16 is used by itself separately, the collecteddust 16 and the separate residue 43 can be put indifferent locations toperform atomization of the dewatering filtration fluid 33.

Third Embodiment

An air pollution control system according to a third embodiment of thepresent invention is explained next. Constituent elements identical tothose in the first embodiment described above are denoted by likereference signs, and explanations thereof will be omitted. FIG. 4A is aschematic configuration diagram of the air pollution control systemaccording to the third embodiment. FIG. 4B is a schematic configurationdiagram of another air pollution control system according to the thirdembodiment. FIG. 4C is a schematic configuration diagram of stillanother air pollution control system according to the third embodiment.In an air pollution control system 10D-1 according to the thirdembodiment, as shown in FIG. 4A, a small second precipitator 35 isprovided on the downstream side of the spray drying device 34 to reducesolid matters.

As the small second precipitator 35, for example, a bag filter or anelectric dust collector can be used. Accordingly, a solid matter 36 canbe reduced from the branched flue gas 18.

Therefore, a gas return line L₁₂ indicated by a dotted line can beinstalled so that the flue gas 18 is joined to the gas supply line L₄ onthe downstream side (this holds true in the following embodiments),other than returning the flue gas 18 to the upstream side of the firstprecipitator 14.

Accordingly, the load on the first precipitator 14 can be reduced.

It can be appropriately changed whether to return the flue gas 18 to theupstream side or the downstream down of the first precipitator 14,according to an amount of generation of the solid matter 36 in the fluegas 18 in the spray drying device 34.

Further, as shown in FIG. 4B, in another air pollution control system10D-2 according to the third embodiment, the small second precipitator35 is provided on the upstream side of the spray drying device 34 toreduce the solid matter 36 beforehand.

Further, as shown in FIG. 4C, in another air pollution control system10D-3 according to the third embodiment, small second precipitators 35Aand 35B are provided on the upstream side and the downstream side of thespray drying device 34 provided in the flue-gas introducing line L₁₁ toreduce the solid matter 36 beforehand. In this case, the gas return lineL₁₂ (indicated by a dotted line in the drawing) can be installed so thatthe flue gas 18 is returned to the downstream side of the firstprecipitator 14, which is preferable. Consequently, the load on thefirst precipitator 14 can be reduced.

Fourth Embodiment

An air pollution control system according to a fourth embodiment of thepresent invention is explained next. Constituent elements identical tothose in the first embodiment described above are denoted by likereference signs, and explanations thereof will be omitted. FIG. 5A is aschematic configuration diagram of the air pollution control systemaccording to the fourth embodiment. In an air pollution control system10E according to the fourth embodiment, as shown in FIG. 5A, a solid-gasseparating spray drying device 50 is used as the spray drying device, toperform spray drying of the dewatering filtration fluid 33. At the timeof spray drying, a solid matter 38 is separated.

As the solid-gas separating spray drying device 50, a cyclone spraydrying device can be used.

FIG. 5B depicts a downflow-type solid-gas separating spray dryingdevice. As shown in FIG. 5B, in a downflow-type solid-gas separatingspray drying device 50, the flue gas 18 is introduced from an upper partof a dryer body 51 to generate a downward laminar gas flow, so that aspray liquid 33 a atomized from above by a spray nozzle 52 is dried.

The flue gas 18 having contributed to drying is discharged from a lowerpart of the dryer body 51, and returned to the gas supply line L₃ of theair heater 13 via the gas return line L₁₂. The solid matter 38 isdischarged from a bottom part of the dryer body 51.

FIG. 5C depicts an upflow-type solid-gas separating spray drying device.As shown in FIG. 5C, in an upflow-type solid-gas separating spray dryingdevice 50, the flue gas 18 is introduced from the lower part of thedryer body 51 to generate an upward laminar gas flow, so that the sprayliquid 33 a atomized from below by the spray nozzle 52 is dried.

The flue gas 18 having contributed to drying is discharged from theupper part of the dryer body 51, and returned to the gas supply line L₃of the air heater 13 via the gas return line L₁₂.

Because the flue gas 18 flows in a direction opposite to a direction ofthe gravitational force, the flue gas 18 is brought into countercurrentcontact with the spray liquid 33 a of the dewatering filtration fluid33, thereby improving drying efficiency of the dewatering filtrationfluid 33.

Further, a small precipitator can be provided on the downstream side ofthe solid-gas separating spray drying device 50 as described in thethird embodiment.

Fifth Embodiment

An air pollution control system according to a fifth embodiment of thepresent invention is explained next. Constituent elements identical tothose in the first embodiment described above are denoted by likereference signs, and explanations thereof will be omitted. FIG. 6 is aschematic configuration diagram of the air pollution control systemaccording to the fifth embodiment. In an air pollution control system10F shown in FIG. 6, a waste-water treatment device 44 is installed inthe dewatering line L₂₁, and after harmful substances and the suspendedsubstance in the dewatering filtration fluid 33 are removed by thewaste-water treatment device 44, treated discharged water 45 is causedto flow into the spray drying device 34 and spray-dried.

The waste-water treatment device 44 includes a unit that removessubstances remaining in the dewatering filtration fluid 33 such asmercury (which was not adsorbed on the gypsum 31), boron, and selenium(hereinafter, “mercury removing unit”), and a unit that removes halogenions such as chlorine ion (Cl⁻), bromine ion (Br⁻), iodine ion (I⁻), andfluorine ion (F⁻) (hereinafter, “halogen-ion removing unit”), toseparate mercury solids 46 and halogen ions 47 from each other.

Substances such as mercury, boron, and selenium are easily soluble inwater, and volatilize when atomized to the flue gas 18. Therefore, thesesubstances are hardly removed by the first precipitator 14. As a unitthat removes these substances, a unit that removes the substances byprecipitation through agglomeration by adding a sulfide coagulation aid,a unit that removes the substances by adsorption (a floating bed) usingactivated carbon, a unit that removes the substances by precipitationthrough the addition of a chelating agent, or a crystallizer can bementioned. The harmful substances are solidified by the mercury removingunit exemplified above, and the solid is discharged to the outside ofthe system.

Because the halogen ions 47 have a property of suppressing adsorption ofmercury on the gypsum 31 at the time of performing a desulfurizationprocess by the desulfurizer 15, it is desired to remove the halogen ions47 from the desulfurization discharged water 30. As the unit thatremoves the halogen ions 47, a concentrating unit using a reverseosmosis membrane, a concentrating unit using an ion exchange membrane, aconcentrating unit using electrodialysis, a distilling unit, or acrystallizer can be mentioned. The halogen ions 47 are concentrated bythe halogen-ion removing unit exemplified above, and the concentrate isdischarged to the outside of the system.

The gypsum 31 adsorbing mercury chloride thereon is first separated fromthe desulfurization discharged water 30 discharged from the desulfurizer15 by the dewaterer 32, and the gypsum 31 is discharged to the outsideof the system. The dewatering filtration fluid 33 after the gypsum 31has been removed is then fed to the waste-water treatment device 44 viathe dewatering line L₂₁, where harmful substances such as mercury,boron, and selenium remaining in the dewatering filtration fluid 33 areremoved by the mercury removing unit. The treated discharged water aftermercury has been removed is fed to the halogen-ion removing unit, wherehalogen ions 47 are removed. The treated discharged water after halogenions have been removed is fed to the spray drying device 34, where thetreated waster water is spray-dried.

The waste-water treatment device 44 does not need to include both themercury removing unit and the halogen-ion removing unit, and any one ofthe units is selected and installed according to the property of thedewatering filtration fluid 33. When mercury is sufficiently removed bythe dewaterer 32 on the upstream of the waste-water treatment device 44and a mercury content in the dewatering filtration fluid 33 is quite lowor mercury is not contained, the process by the mercury removing unitcan be omitted.

Further, the order of a mercury removing process and a halogen-ionremoving process by the waste-water treatment device 44 is notparticularly limited. That is, the halogen-ion removing process can beperformed after performing the mercury removing process, or the mercuryremoving process can be performed after performing the halogen-ionremoving process.

As described above, in the air pollution control system 10F according tothe fifth embodiment, the gypsum 31, which is a bulky matter, is firstseparated from the desulfurization discharged water 30 discharged fromthe desulfurizer 15, fine substances such as mercury, boron, selenium,and halogen ions are removed, and the treated discharged water 45 issubjected to spray drying by the spray drying device 34. By having sucha configuration, as in the second embodiment, the amount of driedparticles generated due to evaporation of the discharged water can bereduced by the spray drying device 34, and an increase in a mercuryconcentration in the flue gas 18 can be suppressed.

Sixth Embodiment

An air pollution control system according to a sixth embodiment of thepresent invention is explained next. Constituent elements identical tothose in the first embodiment described above are denoted by likereference signs, and explanations thereof will be omitted. FIG. 7 is aschematic configuration diagram of the air pollution control systemaccording to the sixth embodiment. In an air pollution control system10G according to the sixth embodiment, as shown in FIG. 7, the flue gas18 is branched from the gas supply line L₃ of the air heater 13, and theflue gas 18 having contributed to spray drying by the spray dryingdevice 34 is returned to the gas supply line L₃ at the same location.

Accordingly, a bypass line provided in the first embodiment is notrequired.

FIGS. 8A to 8C are schematic configuration diagrams of other airpollution control systems according to the sixth embodiment.

In an air pollution control system 10H-1 shown in FIG. 8A, the smallsecond precipitator 35 is provided on the downstream side of the spraydrying device 34 as in the third embodiment, and the gas return line L₁₂indicated by a dotted line can be installed so that the flue gas 18 isjoined to the gas supply line L₄ on the downstream side, other thanbeing returned to the upstream side of the first precipitator 14.Accordingly, the load on the first precipitator 14 can be reduced.

As shown in FIG. 8B, in another air pollution control system 10H-2according to the sixth embodiment, the small second precipitator 35 isprovided on the upstream side of the spray drying device 34, to reducethe solid matter 36 beforehand.

Further, as shown in FIG. 8C, in another air pollution control system10H-3 according to the sixth embodiment, the small second precipitators35A and 35B are provided on the upstream side and the downstream side ofthe spray drying device 34 to reduce the solid matter 36 beforehand. Inthis case, the flue gas 18 can be returned to the downstream side of thefirst precipitator 14, thereby reducing the load on the firstprecipitator 14, which is preferable.

Seventh Embodiment

An air pollution control system according to a seventh embodiment of thepresent invention is explained next. Constituent elements identical tothose in the first embodiment described above are denoted by likereference signs, and explanations thereof will be omitted. FIG. 9 is aschematic configuration diagram of the air pollution control systemaccording to the seventh embodiment. In an air pollution control system101 according to the seventh embodiment, as shown in FIG. 9, the fluegas 18 is branched from the gas supply line L₄ on the downstream side ofthe first precipitator 14, and the flue gas 18 having contributed tospray drying by the spray drying device 34 is returned to the gas supplyline L₃ on the upstream side of the first precipitator 14.

Accordingly, a bypass line provided in the first embodiment is notrequired.

FIG. 10 is a schematic configuration diagram of another air pollutioncontrol system according to the seventh embodiment.

In an air pollution control system 10J shown in FIG. 10, the smallsecond precipitator 35 is provided on the downstream side of the spraydrying device 34, thereby reducing dust in the flue gas 18 havingcontributed to spray drying and returning the flue gas 18 to the gassupply line L₄ on the downstream side of the first precipitator 14.

Regarding the introduction of the flue gas 18, the flue gas 18 isintroduced into the spray drying device 34 according to a difference inpressure drop between the flue gas line and the flue-gas introducingline L₁₁ or introduced by using an inducing pump according to need.

Eighth Embodiment

FIG. 11 is a schematic diagram of a spray drying device of a dewateringfiltration fluid from desulfurization discharged water (a spray dryingdevice) according to an eighth embodiment of the present invention. FIG.13 is a schematic diagram of another spray drying device of a dewateringfiltration fluid from desulfurization discharged water (a spray dryingdevice) according to the eighth embodiment. A specific configuration ofthe downflow-type solid-gas separating spray drying device described inthe fourth embodiment with reference to FIG. 5B is explained below.

As shown in FIG. 11, the solid-gas separating spray drying device 50according to the eighth embodiment includes: the spray nozzle 52 thatatomizes the dewatering filtration fluid 33 from the desulfurizationdischarged water into the spray drying device body 51; an introductionport 51 a provided on the spray drying device body 51 to introduce theflue gas 18 for drying the spray liquid 33 a; a dry area 53 provided inthe spray drying device body 51 to dry the dewatering filtration fluid33 by the flue gas 18; a discharge port 51 b for discharging the fluegas 18 having contributed to drying; and a deposit monitor 60 thatmonitors an attached state of a deposit to the spray nozzle 52.

As the deposit monitor 60, an ultrasonic meter (microwave level sensor)or the like can be used. As the ultrasonic meter, for example, “microrange finder for high-temperature equipment, MicroRanger” (product name:manufactured by WADECO Ltd.) can be used.

FIGS. 12A to 12C are schematic diagrams of monitoring states of adeposit by a deposit monitor.

In FIG. 12A, the deposit monitor 60 that monitors the presence ofdeposit 61 is provided on a side wall of the spray drying device body51, setting sights on a tip section of the spray nozzle 52.

As the deposit 61 of the dewatering filtration fluid 33, an umbrellascale grows at the tip section of the nozzle, which is an ash deposit inthe flue gas 18.

At the time of atomizing the dewatering filtration fluid 33, the sprayliquid 33 a touches the umbrella deposit 61 to coarsen the spray liquid33 a, thereby deteriorating vaporizability of the dewatering filtrationfluid 33.

As shown in FIGS. 12B and 12C, microwaves 63 are generated from thedeposit monitor 60 to measure the distance to a position where thedeposit is generated in a space at the tip section of the spray nozzle52.

FIG. 12B depicts a case that there is no deposit 61. In this case, ameasured distance becomes x, which is determined as normal (no deposit).

In contrast, FIG. 12C depicts a case that the deposit 61 is generated.In this case, a measured distance becomes y, which is determined asabnormal (there is a deposit).

When the growth of ash deposit is detected according to the measurementresult of the deposit monitor 60, a command (*₁₀) to remove the deposit61 is issued.

Other than installing the deposit monitor 60, the presence of deposit 61can be confirmed by visual inspection by an operator.

When visual inspection by the operator is performed, inspection isperformed by using a monitoring inspection hole (not shown) provided inthe spray drying device body 51.

To remove the deposit, there are two methods; which are 1) a method inwhich supply of the dewatering filtration fluid is stopped, thedewatering filtration fluid is replaced by industrial water, andcleaning of the nozzle and the inside of pipes is performed by aspray-nozzle cleaning unit, and 2) a method in which the deposit isforcibly removed by an ash removing unit.

In an air pollution control system 10K, as shown in FIG. 13, replacementby the industrial water is performed by closing a valve V₁₁ to stop thesupply of the dewatering filtration fluid 33 and supplying industrialwater 70 by opening a valve V₁₂. and cleaning of the nozzle and theinside of pipes is performed by the spray-nozzle cleaning unit.

Replacement frequency by the industrial water 70 can be appropriatelychanged such as once per day to once to three times per day, accordingto a deposition degree of the deposit 61. Further, supply time of theindustrial water 70 can be, for example, one hour per once.

At this time, a chemical solution for dissolving the deposit 61 can besupplied.

As the deposit removing unit, a beater (not shown) is provided on thespray nozzle 52 to drop the deposit . The beater can be installed at aposition where spray does not reach.

Alternatively, as the deposit removing unit, a scraper having a circularblade provided on the spray nozzle 52 is operated to cut the deposit 61attached to the tip of the nozzle.

FIGS. 14A to 14C depict a state of removing a deposit by the scraperprovided around the spray nozzle 52.

FIG. 14A is a front elevation of the spray nozzle, and depicts a statewhere the deposit 61 attaches to the circumference of the spray nozzle52. FIG. 14B is a side view of the spray nozzle, and depicts a statewhere the scraper 65 is in a standby state. FIG. 14C is a side view ofthe spray nozzle, and depicts a state where the scraper 65 is activatedand the deposit 61 is crushed and dropped off by the circular blade atthe tip thereof. Dotted line indicates the deposit portion to be droppedoff.

Further, by operating the scraper 65 not only when there is the deposit61 but also frequently to some extent, early dropout of the deposit 61can be effected.

Ninth Embodiment

FIG. 15 is a schematic diagram of a spray nozzle according to a ninthembodiment of the present invention. Constituent elements identical tothose in the eighth embodiment described above are denoted by likereference signs, and explanations thereof will be omitted.

As shown in FIG. 15, the spray nozzle 52 according to the ninthembodiment includes an outer cylinder 67 around the spray nozzle 52 sothat barrier gas 68 is supplied from a supply port 67 a, and air issupplied from the tip section of the nozzle to form an air film, therebysuppressing ash deposition due to soot and dust.

Regarding the supply of the barrier gas 68, the barrier gas 68 isinjected at the same velocity as a jet atomization velocity foratomizing the spray liquid 33 a, thereby preventing generation ofperipheral eddy.

In the ninth embodiment, the scraper 65 is also provided, and thescraper 65 is operated as required to reduce the deposit 61.

In FIG. 15, reference sign 66 denotes an operation handle for thescraper.

According to the spray nozzle 52 of the ninth embodiment, the growth ofthe deposit 61 is suppressed by introducing the barrier gas 68, andstable atomization can be performed by the spray nozzle 52.

Tenth Embodiment

FIG. 16 is a configuration diagram of a spray drying device according toa tenth embodiment of the present invention.

The solid-gas separating spray drying device 50 according to the tenthembodiment further includes thermometers T₁ to T₇ that measure aninternal temperature in the dry area 53, a determining unit 54 thatdetermines the spraying and drying state of the dewatering filtrationfluid 33, and a control unit 55 that adjusts the flue gas 18 or thedewatering filtration fluid 33, when it is determined that spray dryingis not sufficient as a result of determination by the determining unit54.

In the tenth embodiment, the thermometers (T₁ to T₇) are provided atseven positions. However, the present invention is not limited thereto,and the number of thermometers can be appropriately changed according tothe length of the dry area 53.

The thermometers are installed along a vertical shaft line of the dryerbody 51. However, the present invention is not limited thereto, and thethermometers can be installed at any positions, so long as these can beinstalled at positions for confirming the evaporated state.

FIGS. 17A and 17B are relationship diagrams between a distance from thenozzle to the seven thermometers (T₁ to T₇) in the dryer body andmeasured temperatures.

In an evaporation process of the liquid, heat is required fortemperature rise and evaporation of droplets of the spray liquid 33 a.In this case, because heat of the flue gas 18 is used for thetemperature rise and evaporation of the droplets, the temperature of theflue gas 18 decreases. By detecting a decrease in the temperature, thedry condition is determined.

FIG. 17A is a relationship diagram when the dry condition is favorable,and FIG. 17B is a relationship diagram when the dry condition isunfavorable.

In FIG. 17A, a temperature drop stops near T₄, and then the temperaturebecomes constant. This is because there is no droplet of the sprayliquid 33 a.

In contrast, in FIG. 17B, the temperature drop intermittently continuesup to T₇. This is because droplets of the spray liquid 33 a remain in alarge amount.

The determining unit 54 makes a determination based on the aboveresults.

As a result of the determination made by the determining unit 54, whenthe dry condition is favorable, spray drying of the dewateringfiltration fluid 33 is continued.

On the other hand, as a result of the determination made by thedetermining unit 54, when it is determined that the dry condition isunfavorable, the control unit 55 adjusts the flue gas 18 or thedewatering filtration fluid 33.

Specifically, regarding the adjustment of the dewatering filtrationfluid 33, the control unit 55 operates an adjustment valve V₁ to adjusta droplet size of the spray liquid 33 a by increasing or decreasing thesupply amount of the dewatering filtration fluid 33 or by increasing ordecreasing the supply amount of atomized air to be supplied to the spraynozzle 52.

Further, a buffer tank that stores the dewatering filtration fluid 33 ina predetermined amount can be provided to perform adjustment.

Therefore, as shown in FIG. 16, flow amount information (*₁) acquired bymeasuring the flow amount of the dewatering filtration fluid 33 (notshown) is input to the control unit 55, and the control unit 55 adjustsan opening of the valve or adjusts the flow amount of a pump (not shown)based on the information.

The adjustment of the flue gas 18 is performed by controlling anintroduction amount of the flue gas 18.

The adjustment of the introduction amount is performed by controlling anopening of a valve V₂ or a damper by pressure drop adjustment betweenthe flue gas line and the flue-gas introducing line L₁₁.

Further, a series of the flue gas line and the flue-gas introducing lineL₁₁ can be provided, and two or more spray drying devices 34 can beinstalled to adjust the supply amount of the flue gas 18.

Further, when it can be confirmed by the temperature measurement thatthe dry condition is transiently changes from a favorable condition toan unfavorable condition not only by an instantaneous determination butalso by a measurement of a temperature profile over time, the operationdescribed above for eliminating factors to insufficient drying can beperformed.

According to the tenth embodiment, when the dewatering filtration fluid33 acquired by removing the gypsum 31 from the desulfurizationdischarged water 30 discharged from the desulfurizer 15 is spray-driedby using a part of the flue gas 18, the spray drying of the dewateringfiltration fluid 33 is performed while monitoring the temperature statein the dry area. Therefore, the spray-dried state can be stably held andzero liquid discharge of the desulfurization discharged water can berealized. Further, in the spray nozzle 52, the growth of the deposit 61is monitored by the deposit monitor 60, stable operations can beperformed by taking measures to remove the deposit 61 before abnormalatomization occurs.

In the spray drying method of the dewatering filtration fluid 33 inwhich the dewatering filtration fluid 33 is atomized into the dryer body51 and the spray liquid 33 a is dried by the flue gas 18 introducedtherein, the atomization state of the spray nozzle 52 is confirmed todetermine whether atomization of the dewatering filtration fluid 33 isappropriate. When atomization is inappropriate, the spray nozzle 52 iscleaned to remove the deposit 61 attached around the spray nozzle 52,thereby enabling to perform stable spray drying of the dewateringfiltration fluid 33.

Further, the temperature distribution in the dry area 53 inside thedryer body 51 is measured, the dried state is monitored based on thetemperature distribution in a direction of a gas flow, and when thedewatering filtration fluid 33 is dried insufficiently, the supplyamounts of the flue gas and the dewatering filtration fluid 33 areadjusted, thereby enabling to perform stable spray drying of thedewatering filtration fluid 33.

According to the present invention, as the monitoring method of theatomized state, (1) an evaporation state is ascertained based on thetemperature, (2) the growth of the deposit 61 is ascertained byultrasonic waves or the like, and (a) when evaporation is insufficient,the introduced amounts of the flue gas 18 and the dewatering filtrationfluid 33 are adjusted, or (b) when insufficient evaporation is caused bya change in droplet size of the spray liquid 33 a, the spray nozzle 52is cleaned or an ash exhauster is operated, thereby enabling to returnto an appropriate atomized state and perform stable spray drying of thedewatering filtration fluid 33.

REFERENCE SIGNS LIST

10A to 10K air pollution control system11 boiler12 NO_(x) removal equipment13 air heater14 first precipitator15 desulfurizer16 collected dust18 flue gas20 lime slurry21 lime-slurry supply system22 column bottom part23 nozzle24 column top part26 purged gas27 stack30 desulfurization discharged water32 dewaterer33 dewatering filtration fluid34 spray drying device35, 35A, 35B second precipitator44 waste-water treatment device45 treated discharged water50 solid-gas separating spray drying device51 dryer body52 spray nozzle53 dry area54 determining unit55 control unit60 deposit monitor61 deposit63 microwave65 scraper66 operation handle67 outer cylinder68 barrier gas70 industrial water

1. An air pollution control method comprising: burning fuel; recoveringheat of a flue gas; reducing dust in the flue gas after the recoveringheat so as to collect the dust; reducing sulfur oxides in the flue gaswith an lime slurry after the reducing dust; discharging the lime slurryas a desulfurization discharged water; reducing gypsum from thedesulfurization discharged water so as to acquire a dewateringfiltration fluid; spray-drying the dewatering filtration fluid withapart of the flue gas; and supplying the dewatering filtration fluidacquired by the reducing gypsum to the collected dust.
 2. The airpollution control method of claim 1, further comprising: prior to thespray-drying the dewatering filtration fluid, reducing suspended solidsin the dewatering filtration fluid so as to separate a separate liquidfrom the dewatering filtration fluid, wherein, at the spray-drying thedewatering filtration fluid, the separate liquid is spray-dried.
 3. Theair pollution control method of claim 1, further comprising reducingdust in a part of the flue gas prior the recovering heat, wherein thedust reduced flue gas is used as the part of the flue gas of thespray-drying the dewatering filtration fluid.